cpufreq: amd-pstate: Allow users to write 'default' EPP string

The EPP string for 'default' represents what the firmware had configured
as the default EPP value but once a user changes EPP to another string
they can't reset it back to 'default'.

Cache the firmware EPP value and allow the user to write 'default' using
this value.

Reported-by: Artem S. Tashkinov <aros@gmx.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217931#c61


Reviewed-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>